1. Field of the Invention
This invention relates to delayed coking of a petroleum residuum, and more particularly to the production of premium delayed coke, suitable for formation of graphite electrodes for use in electric arc steel furnaces.
Delayed coking of residuum (residual oil) from a petroleum refinery vacuum distillation unit generally produces a coke having a longitudinal coefficient of thermal expansion (CTE) of about 20.times.10.sup.-7 /.degree.C. or greater after graphitization. The CTE of a graphitized coke is an important measure of its suitability for use in the manufacture of electrodes for electric arc steel furnaces. For example, the manufacture of 61 cm diameter electrodes requires a coke having, among other specified characteristics, a CTE of about 5.times.10.sup.-7 /.degree.C. or less, and the manufacture of 38 cm diameter electrodes requires a coke having a CTE of no more than about 8.0.times.10.sup.-7 /.degree.C.
Delayed premium coke is different in many respects from regular delayed coke, and is generally manufactured from specific feedstocks such as decant oil from a liquid catalytic cracker, tar from thermal cracking of regular coker gas oil, pyrolysis tars, or blends of these materials, sometimes with a minor amount of residual oil included in the feedstock. Vacuum residual oil has, prior to this invention, always been considered in the industry as a material incapable of making premium coke when used as the principal ingredient of a delayed coker feedstock, even when coked at conditions which produce premium coke from conventional premium coke feedstocks.
Premium delayed coke, as mentioned above, is a different material from regular delayed coke, which is generally used only as a fuel. Some better quality regular delayed cokes are suitable for use as anodes in aluminum smelting processes, and such cokes are sometimes referred to as metallurgical grade delayed cokes. However, even these better grade metallurgical delayed cokes are a different type of material than what is recognized by and referred to in the industry as premium delayed coke.
Premium coke is worth several times as much as regular coke, and yet the operating costs of a premium delayed coker, not considering the cost of the feedstock, are not greatly different from the costs for a regular coker. Accordingly, any process by which a low value material such as residual oil could be utilized as a premium coker feedstock is much to be desired.
2. The Prior Art
Delayed coking is a routine petroleum refining step used in most cases to dispose of low value residual oils and to convert such residual oils into more valuable products. The operating conditions and process steps of the delayed coking process, both for regular coke and premium coke, are well established and widely known.
The broad process of hydrotreating a residual oil followed by delayed coking of the hydrotreated stream is described in U.S. Pat. Nos. 2,871,182; 2,963,416; and 3,684,688. In addition, U.S. Pat. No. 3,773,653 describes a process of desulfurizing demetalizing residual oil using an ebullated bed reaction zone prior to delayed coking of the hydrotreated stream. More recently, processes including hydrotreating vacuum residuum followed by coking of the hydrotreated stream are described in U.S. Pat. Nos. 3,891,538 and 3,902,991.
Several of the above-mentioned references point out that the coke product obtained has a reduced sulfur and metals content due to the hydrotreating step. Further, several of these references suggest that the coke product is of an improved quality due to the pretreatment of the coker feedstock. However, none of the above-mentioned patents suggests that a premium coke can be obtained by demetalizing and desulfurizing a high sulfur and high metals vacuum residuum and coking the treated residuum in a delayed coker at premium coking conditions. The best coke that can be obtained from residual oil feedstocks, according to the prior art patents, is a coke suitable for use as anodes in the aluminum industry. None of these patents suggests that a premium coke suitable for use in making graphite electrodes for electric arc steel furnaces can be obtained by a process including demetalizing and desulfurizing vacuum residuum followed by delayed coking of the treated residuum.
It is accordingly quite unexpected that a vacuum residuum having high sulfur and high metals content could produce a premium coke. Because of the wide difference in value between regular or metallurgical grade delayed coke and premium delayed coke, it is clearly desirable to be able to produce premium coke from vacuum residuum, and the present invention provides this capability.